Fundamental mode of a conventional optical fiber is composed of two degenerated polarized states. The high-order modes could be two or four degeneracy modes, which depends on the symmetry and degeneracy of the fiber. For example, the LP11 mode and LP21 mode of a step-index circular-core optical fiber are four-fold modes, whereas the LP02 mode is two-fold mode. For the four-fold modes, each polarization state is composed of two modes, the propagation constants of which are equal whereas the mode field profiles of which are different. During the transmission, the two modes would couple owing to the variation of the refractive index of the optical fiber distribution and the environment situation, as a result, energy in the two modes would exchange.
Mode multiplexing techniques are being looked as a novel technique to improve the transmission capacity of optical fibers. Each mode in optical fibers is used as an individual channel in the mode multiplexing systems. Although multiple-input multiple-output (MIMO) can solve the mode coupling problem, this requires complex signal processing. Therefore, uncoupled transmission would be a preferred option. To realized that, each mode in the fiber should be transmitted independently, and avoid the cross-talk caused by the mode coupling. Therefore, degree of degeneracy should be reduced in high-order modes.
Two-mode optical fibers are used widespread in mode converters, mode selective coupler, acousto-optic frequency shifter, and interferometric fiber optic sensor, etc. Two-mode optical fiber refers to a kind of optical fibers that can support the propagation of two-fold LP01 and LP11 modes. This can be achieved by increasing the birefringence of LP11 modes.
Degeneracy of high-order modes can be reduced by applying elliptical-core optical fiber [H. Kubota and T. Morioka. Few-mode optical fiber for mode-division multiplexing. Opt. Fiber Technol., 2011, 17(5): 490-494], or photonic crystal fiber Jin, Z. Wang, and J. Ju. Two-mode photonic crystal fibers. Optics Express, 2005, 13(6): 2082-2088. . Deformation can be induced to both the high-order modes and the fundamental mode.